A stator and a rotor of a motor or a generator, gears and rotation shafts of a reduction gear, beam members of a transport machine such as an automobile, a frame structure of a building, a large-sized mechanical structure, a structure for fixing the same, and so on generally vibrate. A damping technique for depressing vibration of such a structure, by providing a damping member having a hollow body in which powder/particle materials such as grain or powder are filled in a closed space, on the structure which is vibrating, has been already developed. This technique has been actually adopted, in a field where the vibration cannot be overcome by a technique using damping material such as elastic material or a vibration absorbing device, which has been heretofore widely employed. Such a technique has been proposed in Patent Documents 1 and 2, and so on.
In the technique disclosed in Patent Document 1, vibration of a motor having various kinds of frequencies and level characteristics is intended to be reduced, by fixing a damping member filled with powder/particle materials to the motor. Moreover, in the technique disclosed in Patent Document 2, cavities are formed in a timing pulley which is meshed with a timing belt for transmitting a motive power, and powder/particle materials are movably disposed in the cavities, thereby to damp vibration generated by the mesh between the timing belt and the pulley, and to reduce noises.
By adopting these techniques, it is certainly possible to obtain the damping effect. However, the damping effect by using the powder/particle materials has such a feature that it has non-linear characteristics, and therefore, there is a problem that by simply filling the cavities with the powder/particle materials, reliable damping effect cannot be obtained, depending on conditions.
Moreover, by adopting these techniques, sufficient damping effect cannot be obtained against vibrations with small amplitudes. The damping effect by using the powder/particle materials is realized by mutual collisions, deformations, and frictions of the powder/particle materials which are generated, when the powder/particle materials move with vibration. Particularly, in case where vibration in a vertical direction is an object to be damped, the powder/particle materials must move against gravitational force. Therefore, there has been a problem that vibration acceleration of 1 G or more is required for obtaining the damping effect.